As a technique for wastewater treatment, a method using a membrane module that separates impurities from water is known. In the method using such a membrane module, separation membranes of the membrane module need to be cleaned, because impurities are accumulated on the separation membranes. The separation membranes are cleaned, for example, using bubbles. Examples of the technique using bubbles include a membrane module system that uses a pulsed gas lift pump (see Japanese Patent No. 4833353).
The membrane module system disclosed in this document is immersed in a liquid during use. The membrane module system supplies, to a membrane module, bubbles generated by continuous supply of pressurized gas and a high speed gas-liquid two-phase flow of feed liquid, thereby scouring the surfaces of permeable hollow fiber membrane bundles in the membrane module. The high speed gas-liquid two-phase flow contains a high-speed moving liquid and many independent small-diameter bubbles therein.